More precisely, the present invention relates to one of the component elements thereof, i.e. a centrifugal compressor wheel that comprises a hub, a web extending radially from the hub, and a plurality of blades carried thereby.
Below, the adjectives “axial” and “radial” are considered relative to the axis of rotation of the compressor wheel.
Such a centrifugal compressor wheel, well known from elsewhere, co-operates with a radial diffuser to compress the air that enters the compression stage axially prior to leaving it radially.
In known manner, each of the blades extends from a leading edge to a trailing edge and it presents a pressure side and a suction side.
While the compression stage is in operation, particularly but not exclusively when fitted to an aircraft turbine engine, such as a helicopter gas turbine, the compressor wheel tends to become eroded, in particular because particles such as sand become ingested into the compression stage.
After several hours of operation, it is generally found that erosion profiles are present, in particular in the form of retreats of the leading edges and of furrows at the roots of the blades on the pressure sides and extending towards the trailing edges. In other words, the presence of furrows in those locations of the wheel is the result of the wheel being eroded.
The retreat of the leading edges may give rise to degraded performance and degraded aerodynamic stability of the compressor, and also to degradation in the mechanical strength of the blades. Furthermore, the furrow degrades the mechanical strength of the impeller disk. Erosion of the leading edges is easily detected by conventional means (a camera looking into the air inlet of the engine) and it can happen that it is not as great as the furrow type erosion. It is therefore necessary also to inspect for furrow type erosion, such that once the compressor wheel is excessively eroded by the furrow, it is necessary to change the wheel.
Generally, the erosion profile is very fine and not very visible, so it is difficult to determine quickly whether the amount of erosion that has appeared is or is not acceptable.